This invention relates to torque wrenches such as an impact wrench and a hydraulic wrench.
In utilization of an impact wrench, it is necessary to detect the striking state in order to accurately control the tightening torque or accurately grasp the number of tightened nuts. Conventionally, the striking state is detected by detecting the reverse rotation of the main shaft of an air motor using a sensor mounted to this main shaft. However, since this method requires that the sensor to detect the forward and reverse rotations of the main shaft to be disposed within the impact wrench, it has disadvantages that the machine would be enlarged and its cost would be increased.
For solving such a problem, a system can be used which detects an admission pressure and detects the striking state as the impact wrench is activated when the admission pressure falls below a reference level. With the use of this system, however, it is not possible to discriminate whether the impact wrench is in free running (no-loaded rotating) state or striking state. This is because the difference between the pressures generated in free running state and striking state is smaller than a variation in pressure which is inevitably caused in the admission source, thus making it impossible to distinguish one state from the other.